1. Field of the Invention
The present invention relates to a method and to apparatus for transmitting information relating to the operation of an electrical device disposed in a borehole, e.g. such as a submersible pump for pumping the hydrocarbons coming from the underground formations through which said borehole passes. The information relating to the operation of the pump includes, for example, outlet and inlet pressures, flow rate, fluid density, motor temperature, vibration, or any other information that is suitable for controlling and monitoring the operation of the pump.
2. Description of Prior Art
Because formation pressure decreases after a certain period of production, it is necessary to make use of artificial means to bring hydrocarbons from the bottom of a well to the surface, and this is done by means of pumps driven by high power electric motors (e.g. of the order of 300 horse power). Such pumps require electrical power at high voltage and at high current, thereby giving rise to high cost in power consumption, and also requiring equipment for conveying electrical power from the surface to the motor that drives the pump.
Since, by definition, such pumps are not directly accessible, they must be monitored remotely from the surface.
Known monitoring techniques have been developed (see U.S. Pat. Nos. 3,340,500 and 4,901,070 for example) that make use of the electrical power wire or cable connection connecting the electric motor of the pump to power supply means on the surface. In general, the power supply is of the three-phase type and comprises three electrical cables or wires, each for one of the phases, the three wires being connected to ground at the electric motor driving the pump by means of a "star" or "Y" junction. At the surface, the three wires are connected to electrical power supply transformers. Parameters relating to the operation of the pump, or of the electric motor, are picked up by at least one sensor disposed in proximity to the pump or the motor and is connected to at least one of the phase wires for powering the motor via a resistive connection, i.e. a direct wire-to-wire junction at the pump/motor assembly. A second resistive connection at the surface serves to connect said phase wire to a measurement device suitable for receiving and/or processing information directly. It is usual to provide a filter (of the RC type) between the reception means on the surface and the resistive connection.
Information is then transmitted by injecting DC from the reception device on the surface to the sensor which in turn sends information back to the reception means on the surface by using the corresponding phase wire to convey the information.
That known method and apparatus give rise to major drawbacks.
Firstly, the resistive and direct junction constitutes a hazard for the user when connecting the reception device for processing the information. The phase wire conveys high tension electricity (at about 2400 volts). In order to reduce considerably the risk of electrocution, circuits are provided that constitute a safety barrier, thereby not only increasing cost, but also failing to eliminate the hazard completely. Further, the use of apparatus of this type is being impeded by the psychological reticence of users coming to connect a device to a line that is carrying high tension. Furthermore, in the event of the filter or the safety circuits failing, several thousand volts from the phase wire are injected into the information-receiving device (such as a personal computer), with the resulting hazard for the operator, not to mention the damage to said computer.
Furthermore, it is usual to perform tests to verify that the power supply wires or cables are themselves well isolated, electrically. To do this, a high DC voltage (of the order of 500 volts or more) is injected from the surface. Nevertheless, that isolation test is of poor reliability given the leaks that occur via pressure gauges constituted by resistances.
Attempts have been made to remedy that drawback (see for example U.S. Pat. No. 4,157,535 or 4,523,194) by providing means for disconnecting the circuit including the downhole pressure gauge so as to enable the isolation test to be performed. Such circuits are relatively complicated, thereby increasing cost, and their components disturb the operation of the pressure gauge circuit.
Proposals have also been made to transmit information relating to the operation of the pump via a direct connection, using an electric cable that is separate from the pump power supply cables. The distinct cable for information transmission is connected directly to the sensor and receives DC or AC.
That known means suffers from drawbacks.
Firstly it is very expensive given its structure.
Equally, the presence of a distinct cable makes it necessary to provide through-holes for said cable at each sealing member, thereby increasing the risk of leaks.
Finally, it is very difficult if not impossible to install the above-mentioned known apparatus on existing equipment that is already in place without major modification, and that increases installation cost.